1. Field of the Invention
The present invention relates generally to holographic elements and more particularly to such a holographic element in which a plurality of regions, which include a region developing in dependence on polarization directions of an incident light thereon a phase difference between polarization components of a transmitted light and are different as a whole in the phase difference between the polarization components of the transmitted light, are formed on a substrate substantially periodically to provide different diffraction efficiencies in dependence on polarization components of the incident light.
This application is based on Japanese Patent Application No. Hei 11-99463, the contents of which are incorporated herein by reference.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
A great number of holographic elements for switching light paths depending upon polarization directions of an incident light have been used in the field of optical communication and optical discs. For example, a wave plate which is formed by laminating plural surface relief type holograms that switch a plane of polarization of an incident light is disclosed in Japanese Unexamined Patent Application, First Publication, No. Sho 62-211603. Furthermore, a diffraction grating type polarizer which is formed by arranging and joining a double refraction material and a second optical material via adhering layers on a first optical material of the polarizer is disclosed in Japanese Unexamined Patent Application, First Publication, No. Hei 9-292520. One conventional holographic element of this kind is shown in FIG. 17. This figure is a crosssectional view of the conventional holographic element which has a polarization dependency and diffracts only a specific polarization component.
This holographic element is constructed, as shown in the FIG. 17, such that strip-like proton exchange regions 2 each extending in a direction perpendicular to the drawing and proton non-exchanged regions 3, which are exposed substrate regions extending along the adjacent proton exchange region 2, are alternately formed in a substrate 1. This substrate is made of lithium niobate which is a birefringent crystal having different refractive indices depending on the polarization directions of an incident light. The proton exchange regions 2 are formed by immersing the lithium niobate substrate 1 whose surface is covered by a mask having openings in an acid containing protons thereby to cause the proton ions and the lithium ions to be exchanged by their mutual diffusion through the openings of the mask. The proton exchange region 2 and the proton non-exchanged region 3 have different birefringent characteristics from each other. These two regions 2 and 3 can produce, for mutually orthogonal polarization components (hereinafter referred to as xe2x80x9cP polarizationxe2x80x9d and xe2x80x9cS polarizationxe2x80x9d) of lights vertically incident on the substrate, different phase differences between orthogonal polarization components of the transmitted lights.
A phase adjusting film 4 is formed on each proton non-exchanged region 3. The phase adjusting film 4 is provided for adjusting the phase difference between the regions 2 and 3 to a desired value and is made from an isotropic medium. In FIG. 17, the phase adjusting film 4 is formed only on the proton non-exchanged regions 3, but it may also be formed on either the proton exchange regions 2 or the proton non-exchanged regions 3, or on both the regions 2 and the regions 3.
As shown, for example, in FIGS. 6 and 7, the phase adjustment can be made such that the phase of the transmitted lights in specific regions changes by 180 degrees for the P polarization but that the phase does not change in any regions for the S polarization. A polarization dependent holographic element which selectively diffracts the P polarization component can thus be obtained.
This phase adjustment can be made using parameters given below in addition to the adjustment parameters relating to the phase adjusting film 4 in accordance with the following formulas (1) and (2):
Dhxc2x7(Nhpxe2x88x92Np)+Dxc2x7(1xe2x88x92N)=2xc2x7nxc2x7xcex+xcex12xe2x80x83xe2x80x83(1)
Dhxc2x7(Nhsxe2x88x92Ns)+Dxc2x7(1xe2x88x92N)=2xc2x7mxc2x7xcexxe2x80x83xe2x80x83(2)
where m and n are positive integers, Np a P polarization refractive index of the proton non-exchanged region 3 which is a part of the lithium niobate substrate 1, Ns an S polarization refractive index of the proton non-exchanged region 3, Dh a thickness of the proton exchange region 2, Nhp a P polarization refractive index of the proton exchange region 2, Nhs an S polarization refractive index, D a thickness of the phase adjusting film 4 and N a refractive index of the phase adjusting film 4.
The above-described holographic element, however, has a problem that it is rather expensive due to the fact that it requires an expensive birefringent crystal of lithium niobate and a dedicated treatment facility for obtaining the proton exchange regions 2 by means of acid treatment.
Furthermore, a phase adjustment is made using a double refractive film that is formed by the diagonal vapor deposition in a diffraction grating type polarizer, and the double refractive film has a double refractiveness dependent upon a material and substrate. This diffraction grating type polarizer is disclosed in Japanese Unexamined Patent Application, First Publication, No. Hei 7-5316.
To solve this problem, holographic elements which do not employ a birefringent material have been developed. One of conventional holographic elements of such type is disclosed in Japanese Unexamined Patent Application, First Publication, No. Hei 2-96103. The structure of this holographic element is characterized in that a plurality of regions, each having a concavo-convex or a an corrugated surface configuration of a pitch less than a half of the wavelength of an incident light, are formed in a surface of a substrate made of an isotropic medium in such a manner that the directions of pitch of those regions differ from each other. The concavo-convex configurations are obtained by forming grooves in the substrate by means of a photolithographic technique or an etching technique. In a holographic element of this type, the phase adjustment is carried out mainly by adjusting a depth of the grooves. In general, the greater the difference in depth of the grooves between the regions, the greater the phase difference.
The holographic element of the above structure which does not make use of a birefringent material, however, has such a problem that since the grooves must be formed with a depth substantially greater than the pitch of the corrugated configuration in order to obtain a desired phase difference, its productivity is rather low.
It is therefore an object of the present invention to provide a holographic element that can be manufactured without the use of an anisotropic material and comprises recesses and projections of moderate depth and height which correspond to a pitch of the concavo-convex configuration.
To solve the aforesaid problems, according to a first aspect of the present invention, there is provided a holographic element in which a plurality of regions, including a region for developing in response to an incident light thereon a phase difference between polarization components of a transmitted light in dependence on polarization directions of the incident light and having, as a whole, different phase differences between polarization components of the transmitted light, is formed substantially periodically in a substrate to provide different diffraction efficiencies depending upon polarization components of the incident light, wherein at least one of the plurality of regions within one cycle of the substantially periodically formed regions is a region in which a concavo-convex configuration having a one-dimensional periodic structure with a pitch equal to or less than a wavelength of the incident light is formed, and wherein a multitude of layers made of isotropic media having different refractive indices are laminated to form a multilayer film on the region with the concavo-convex configuration in such a manner that a refractive index of the multilayer film varies periodically across a thickness thereof.
As a second aspect of the present invention, the polarization directions may be perpendicular to each other.
As a third aspect of the present invention, in the holographic element, a phase adjustment may be made in such a manner that an amount of change in phase of a polarization component in a specific direction among the polarization components of the transmitted light is constant throughout the plurality of regions.
As a fourth aspect of the present invention, the substrate may be a transparent substrate and in that the region formed with the concavo-convex configuration has such a structure that the multilayer film is laminated on projections and recesses formed in the transparent substrate at a pitch equal to or less than the wavelength of the incident light.
As a fifth aspect of the present invention, in the holographic element, at least one of the regions within one cycle of the substantially periodically formed plurality of regions may be a planar region on which the multiplayer film is deposited.
As a sixth aspect of the present invention, in the holographic element, a phase adjustment may be made within one cycle of the substantially periodically formed plurality of regions by adjusting a thickness of the substrate for each region.
As a seventh aspect of the present invention, in the holographic element, a direction of the substantially periodic formation of the plurality of regions and a direction of pitch of the one-dimensional periodic structure in the region with the concavo-convex configuration may be different from each other.
As a eighth aspect of the present invention, the plurality of regions may include, within one cycle of the substantially periodically formed plurality of regions, a plurality of regions formed respectively with concavo-convex configurations whose one-dimensional periodic structures have mutually different directions of pitch.
As a ninth aspect of the present invention, the holographic element may include, within one cycle of the substantially periodically formed plurality of regions, two or more regions formed respectively with concavo-convex configurations whose one-dimensional periodic structures have directions of pitch which agree with the direction in which the plurality of regions are substantially periodically formed, wherein the pitch of the one-dimensional structures gradually increases or decreases in the direction in which the plurality of regions are substantially periodically formed.
According to a tenth aspect of the present invention, the recesses in the concavo-convex configuration formed in the substrate may be grooves formed in the substrate.
With the structure according to the present invention, at least one region within one cycle of a plurality of regions is selected to be a region in which a concavo-convex configuration having a one-dimensional structure of a pitch not greater than the wavelength of an incident light is formed and a multitude of layers made of isotropic media of different refractive indices are laminated to form a multilayer film on the regions formed with the concavo-convex configurations in such a manner that the refractive index in the multilayer film varies periodically across a thickness thereof, so that it is possible to utilize the thickness and the refractive indices of the layers of the multilayer film as parameters for adjusting a phase difference between the polarization components of the transmitted light.
Thus, as compared to the conventional case where a difference between high and low levels in the concavo-convex configuration can only be used as parameters for the phase adjustment, a variety of parameters is available for such phase adjustment in the present invention, as a result of which a required phase difference can be obtained by a proper magnitude of difference between high and low levels in the concavo-convex configuration. The manufacturing of the holographic element according to the present invention can thus be relatively easy.